The present invention relates generally to a method of operating a coherent radar, and more particularly, to a method of operating an agile beam coherent radar in an increasing target environment without reducing the performance thereof.
Surveillance radars of the early warning or advanced warning variety are generally operative to perform a variety of mission tasks or functional modes, such as searching, tracking and the like in a multiple target environment. Usually, the performance of a radar is measured in accordance with the duration of its frame time, which is the time during which all of the required modes are to be performed by the radar for as many targets detected and searchable areas designated. In a single target mode operation, which will be described in greater detail herebelow, the frame time may shrink or swell depending upon the density of the target environment in which it is operating. For an airborne surveillance radar, the lowest fundamental periodic cycle of the frame time is conventionally based on the time the radar has to detect each new target as it becomes visible over the horizon, which may be on the order of 10 seconds for contemporary systems. For this example, should the frame time swell in excess of the 10 second period requirement because of an increase in the target environment, the potential for missing a target during the searching modes will be commensurately increased therewith.
The aforementioned shortcoming results from the single target mode operation; however, this problem may be alleviated by transferring to an interleaved mode operation within the frame time of the radar. To better understand these operational modes, it will be necessary to probe into what is occurring within the frame time. For each target and functional mode associated therewith, there is an operational time duration, known as the mode dwell time, which is the time required by the radar to illuminate the target, receive radar data therefrom, and extract information from the radar waveform necessary to fulfill the prespecified function of the mode. In general, a mode dwell time may include on the order of three to five coherent integration times or radar looks, with each look including a predetermined number of interpulse periods for coherent integration. For example, for a 64-point Fast Fourier Transform (FFT), there is required 64 interpulse periods for each coherent integration time. In addition, there is a mode updating frequency which is the frequency at which information is desired from a functional mode or task, like the track update frequency on a particular target, for example, which may be dependent upon the speed, range . . . etc. of the particular target. For the search mode, in most cases, the mode update frequency corresponds to the largest allowable frame time.
For the most part, the aggregate of the search mode dwell times consumes the largest portion of the frame time and may be as much as 80% or more thereof. This, of course, leaves a small percentage of the frame time to perform all of the other mode functions required for the detected targets. In the single target mode of operation, the predetermined number of mode dwell times are spread out along the time axis of the remaining portions of the time frame. In those instances in which the target density increases to a point of overloading the allowable frame time, some interleaving of mode dwell times becomes necessary in order to stay within frame time mission requirements.
To understand this process of mode interleaving, it is convenient to think of the frame time being partitioned into some number of time quantizations or slots which may or may not be of equal duration. As indicated above, a majority of the time slots are filled by dedicated regional mode searches so that there may be only a small percentage of empty time slots in which to insert the other required mode dwell time operations for the detected targets in accordance with the update frequency thereof. When there is more than one mode which is scheduled to be executed during the same time slot as a result of mode update frequency, for example, the time slot becomes under competition. In general, this time slot under competition is greater in duration than the mode dwell time, but less than the time for a separate mode dwell for each competing target. A method of resolving the competition is to interleave or merge two or more mode dwell times into a single time interval. As a result of the scheduled interleaving of the mode dwell times, the overall frame time may be maintained within the bounds of the frame time mission requirement.
The concept of merging two or more mode operations into a given time interval of the frame time involves primarily the interleaving of the radar time events associated therewith. Remember that each of the mode dwell times comprises a number of coherent integration times or radar looks each including a large number of interpulse periods forming a train of transmission and echo pulses corresponding thereto. Thus, it is these trains of pulses and corresponding switching times which have to be interleaved during the merging operation. Consequently, it is not a simple matter of merely scheduling two or more mode operations for the same designated interval of time in the frame time. There must be some order or priority selected for the time event patterns associated with each of the mode operations so as to eliminate the possibility of eclipsing or event coincidence, the consequences of which would, more than likely, result in a loss of vital information. This interpulse interleaving should be adaptable from one frame time to another because of the changing target environment and system mode requirements. That is, since the interpulse scheduling is applied on a coherent look basis, everytime there is a new mode dwell time operation to be merged in the same time interval of the frame time, there will have to be a new scheduling arrangement of interpulse events. Another difficulty which may arise in some instances is that the multiple pulse trains being merged may each have a different pulse repetition frequency (PRF) in which case the pulses of a train with a short interpulse period will reappear more rapidly than the pulses of a train having longer interpulse periods. As a result of this phenomenon, a series of respective pulse trains running through each other is created during the merge which may give rise to a higher probability of eclipsing.
An example of a method of simply interleaving pulse trains of a radar for increasing the data rate thereof is disclosed in a French Patent Application No. 2,412,852; entitled "Improvements To Pulse Doppler Radars"; filed Dec. 22, 1977 by Jean-Louis Gaston Bossenec et al. and assigned to Laboratories Central de Telecommunications, France. This method involves the use of simultaneous interleaved pulse trains whose pulse repetition frequencies (PRF's) are constructed from irrational numbers which prevent absolute alignment of the transmit pulses in time throughout a coherent integration period. One drawback is that upon reception, echo pulses of PRF differing trains may partially eclipse each other. The eclipsing is minimized by the substitution of calculated pulse estimates for the destroyed pulses during post-processing operations. The method appears to be intended for reducing the time for determining unambiguous target range and/or doppler for several ambiguous measurements from the same target. Application to a beam agile radar for a multiple target environment is not mentioned.
The method described herebelow in the preferred embodiment section provides a technique which is intended to avoid the pulse conicidence or eclipsing drawback in the merging operation of multiple functional modes in the same designated time interval of a frame time by a technique of scheduling the interpulse interleaving events in accordance with a selected set of PRF's adaptable to the instantaneous target environment and desired mode operation associated therewith.